Plasma normally contains no free hemoglobin ie, no hemoglobin that is not contained within erythrocytes.

Due to normal blood draw procedures, a small amount may be present in normal people, and when detectable, the total plasma hemoglobin and the subcomponent, oxyhemoglobin, are both reported.

Significant amounts of free hemoglobin occur in plasma following hemolysis for any reason. This might result from a transfusion reaction or mechanical fragmentation of red blood cells due to instrumentation, surgical procedures, or mechanical devices. Patients requiring support from extracorporeal membrane oxygenation (ECMO) or centrifugal ventricular assist devices (cVAD) are commonly monitored for trends in plasma free hemoglobin levels to assess for increasing hemolysis. Sharp spikes in plasma hemoglobin levels can indicate pump disruption. However, plasma hemoglobin can be artifactually increased due to a traumatic blood draw or prolonged exposure to post-draw red blood cells. Additionally, bilirubin interferes substantially with the ability to calculate total plasma hemoglobin levels and gives spurious and unreliable results, a difficulty frequently encountered in serially tested patients. When this occurs, the oxyhemoglobin level tends to show less interference and it is the only analyte reported in the presence of increased bilirubin (>5 mg/dL). When using trending data, total plasma hemoglobin and oxyhemoglobin levels are not interchangeable and should be compared within their subgroups only.

Plasma should be spun down and transferred to a secondary aliquot tube within 2 hours of draw as falsely elevated levels result if left longer than 2 hours due to post-draw RBC lysis.

Bilirubin is a known interference with this assay. Elevated bilirubin levels significantly interfere with the quantitation of the total plasma hemoglobin. In the presence of elevated bilirubin (>5 mg/dL), only oxyhemoglobin will be reported. Trending data should be compared within analytes (ie. total hemoglobin to total hemoglobin; and oxyhemoglobin to oxyhemoglobin) as these represent different components in the blood. Therefore, in the presence of bilirubin, the trend can only be compared with baseline oxyhemoglobin levels.

If bilirubin is >5 mg/dL, this comment will be added to the report:

Total plasma hemoglobin not reported due to elevated bilirubin interference. Oxyhemoglobin levels are not affected as significantly by bilirubin levels and are more reliable in this setting. If the plasma hemoglobin test is being utilized in a serial fashion, results should be compared within analyte type (eg, oxyhemoglobin to oxyhemoglobin levels for different time points) and using general trends. Because some variability in this test exists, multiple time points should be used to monitor trends and to confirm wide swings in levels. Because pigmented substances may cause nonspecific interference, clinical correlation is required to establish significance of individual test results.